1. Field of the Invention
The present invention relates to magnetic circuits with opposing permanent magnets, and specifically relates to magnetic circuits with opposing permanent magnets suitable for magnetic resonance imaging apparatuses (also referred to MRI) which obtain images of bodies by utilizing magnetic resonance.
2. Description of Related Art
For magnetic field generating devices using permanent magnets for magnetic resonance apparatuses, the use of magnetic circuits with opposing permanent magnets utilizing rare earth magnets is well-established. As detailed below, such magnet opposing-type magnetic circuits include permanent magnet units and a yoke for allowing a magnetic flux from the magnets to pass therethrough, and generally also include pole pieces that are preferably made of soft magnetic material such as soft iron, provided on surfaces of the permanent magnet units in order to generate a uniform magnetic field in the gap between the permanent magnet units.
Japanese Patent Provisional Publication No. 2000-51175 A, Japanese Patent Provisional Publication No. H8-339916/1996 A, all of which are herein incorporated by reference, and the like teach a method for assembling such magnetic circuits with opposing permanent magnets. As disclosed in these documents, when assembling magnetic circuits with opposing permanent magnets, a permanent magnet unit is arranged on a base yoke in a plate-shaped, disk-shaped or the like, followed by arranging a pole piece on an upper face of the permanent magnet unit.
Before the pole piece is arranged on the permanent magnet unit, the permanent magnet is very susceptible to a self-generated magnetic field (demagnetization field). Therefore, if the coercive force of the permanent magnet is weak, then magnetism may be reduced due to the demagnetization field, resulting in reduction of the magnetic field generated by the permanent magnet, and there is a possibility that the obtained magnetic field will be less than the design value even after a pole piece is arranged. Thus for the permanent magnet used in the magnet circuit, it has been necessary to use a permanent magnet whose coercive force is sufficiently high. However, special materials and technologies are required for manufacturing a magnet having a high coercive force. Nd2Fe14B-based rare earth magnets are used as the permanent magnets for magnetic circuits with opposing permanent magnets currently widely used. However, further adding elements such as Dy and Tb is required in order to raise the coercive force of the magnets. The use of these added elements causes an increase in the cost of the magnet, resulting in an increase in costs of the magnetic circuit.